1. Field of the Invention
The present invention pertains to a method for hydraulically fracturing an earth formation from an inclined or deviated wellbore to improve the conductivity of the fracture.
2. Background
In the production of fluids from subterranean formations, it has been a long-standing practice to hydraulically fracture the formation from a wellbore to enhance the flow of fluids from the formation into the wellbore. It has been recognized for some time that the propagation of a fracture in an earth formation proceeds generally in a plane which is normal to the direction of the minimum principal stress existing in the formation. In a majority of cases, in deep wellbores, the direction of this stress is horizontal and, accordingly, the fracture is a generally vertical propagating fracture in a plane perpendicular to the minimum stress. In certain shallow wells, depending on formation characteristics, the fracture may propagate in a generally horizontal plane if the compressive stresses are greater in the horizontal rather than the vertical direction.
For generally vertical wellbores, the overall fracture length and direction of propagation can be easily controlled. For example, in a cased wellbore, if the direction of the minimum principal stress is determined, the wellbore casing may be perforated along a line which lies in a plane which is normal to the direction of the minimum stress. Thus, the injection of fluids through the perforations will initiate a series of fractures which will eventually link up and become a single vertically extending fracture, generally in a plane normal to the minimum stress.
In recent years the search for hydrocarbon reservoirs has required the drilling of many inclined or deviated wellbores which intersect the direction of the minimum principal stress in the formation at an angle other than 90 degrees. The development of suitable fractures extending from such wellbores has not been particularly successful. Some wells have been drilled with a so-called "S" shape, that is, starting out vertical, then being inclined, and finally resuming a generally vertical direction in the zone that is to be fractured in order to avoid problems associated with poor fracture propagation from deviated or inclined wells. U.S. Pat. No. 4,669,546 to Jennings, Jr. et al and issued June 2, 1987 describes a method for improving vertical fractures of inclined wellbores by providing a series of in-line openings along the low side of the wellbore casing. This technique provides suitable fractures in only very limited cases, that is, primarily where the wellbore extends in the plane of the direction of the maximum principal stress. Other factors often dictate the direction of a wellbore and the likelihood of having a wellbore extending in such a preferred direction is very low.
Certain efforts have been made to improve on the method described in the Jennings, Jr. et al patent such as described in the paper published by the Society of Petroleum Engineers, Richardson, Texas, under No. SPE 19722 entitled "On Fracture Design of Deviated Wells" by C. H. Yew, Joseph H. Schmidt and Yi Li. This paper prescribes, among other things, the optimum angle with respect to the wellbore axis for providing perforations in cased wellbores to initiate fractures which will provide greater conductivity.
However, in the development of wellbore fractures from inclined wells, it has been discovered that the near wellbore formation stresses tend to reduce the fracture dimensions and the fracture does not grow in length or height until it has turned to lie in the plane which is normal to the direction of the minimum principal stress. Even though the technique described in the above-referenced paper optimizes the location of casing perforations for cased wellbores, the plane of the fracture will still undergo some degree of turning as it grows in the vertical direction. Accordingly, the fracture in the near wellbore region is of smaller cross-sectional area, may be subject to relatively high closing stress and may form a point of throttling or choking of the flow of fluids between the formation and the wellbore.
One problem which has been discovered is that if the fracture is not suitably held open by the injection of a proppant, the fracture will reclose in the near wellbore region and force proppant and fluids into the main body of the fracture upon relaxation of pumping pressure. This will result in costly refracturing operations to reopen the fracture and possibly result in a poor completion. Accordingly, the present invention is directed to an improved method of completing a fracturing operation in a subterranean formation where such fracturing is carried out primarily from deviated or inclined wellbores and which operation overcomes some of the problems associated with prior art efforts to fracture formations from inclined wellbores.